Cyclic communication method via a bus

ABSTRACT

A method for cyclical communication between communications stations controls or surveys a technical process, through a bus. Communications relations, which are provided for the communication stations are executed during each bus cycle of predetermined duration and if a communications relation is disturbed, its repetition is scheduled for a succeeding bus cycle and the disturbed communications relation is acknowledged with a special acknowledgement code.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/EP01/08820 filed on 30 Jul. 2001 and EuropeanApplication No. 001 17 177.6 filed on 10 Aug. 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for cyclic communication betweencommunication stations provided for controlling or monitoring atechnical process, via a bus, in which communication sessions plannedfor the communication stations are executed during in each case one buscycle.

Such a communication method is known from the standard EN 50 170 orPROFIBUS standard. The PROFIBUS is a so-called field bus which is usedfor communicatively linking communication stations provided forautomating a technical process. A communication station at this or asimilar bus is, e.g., a so-called stored-program control (SPC). Afurther communication station at the bus is, e.g., a so-calleddecentralized peripheral device to which external sensors or actuatorscan be connected for controlling or monitoring the technical process.

Controlling a technical process frequently also includes closed-loopcontrol tasks. In this context, closed-loop control comprises picking upa measurement value from the technical process and outputting controlinformation to the technical process. Both the picking-up of themeasurement value and the outputting of the control information isusually cyclic. Since the measurement value is thus available notcontinuously but only in each case at the time when it is picked up,i.e. at the sampling time, this is a sampled-data control system, thequality or stability of which is primarily dependent on the equidistanceof the sampling times.

Frequently, the measurement value is picked up from the process by afirst communication station and processed by a second communicationstation. Using the measurement value, this second communication stationalso generates the control information to be output. The controlinformation is then output to the process by a third communicationstation. The distance between two sampling times, and, therefore, thesampling frequency is thus determined by the duration of thecommunication between the respective communication stations.

To ensure equidistance of the sampling times, a constant bus cycle timeis provided, e.g. in the PROFIBUS. The bus cycle time is the timeinterval within which all cyclic communication sessions planned for thecommunication stations connected to the bus are executed exactly once. Acommunication method with constant bus cycle time is known, e.g., fromGerman patent application 199 39 182 (date of application 20, Aug.1999).

A communication comprises the transfer of a message via the bus from thetransmitting communication station to the receiving communicationstation. The time required for transferring a message is essentiallydetermined by the volume of data transferred. The volume of data ofcyclic communications, however, is essentially constant. Thus, anapproximate equidistance between the individual communications isobtained with a constant bus cycle time. The approximate equidistance ofthe individual communications is accompanied by an approximateequidistance of the sampling times because the measurement value pickedup from the process is a data item of a communication or, respectively,a message.

To ensure actual equidistance, the bus cycle time is longer than thetime which would be required for executing all planned communications.The additional time is available as reserve for message retransmissionsand so-called acyclic messages. If no message retransmissions arerequired in a bus cycle or there are no acyclic messages ready fortransfer, the system still waits until the predetermined bus cycle time(including the spare time) has elapsed before it begins the next buscycle. This results in a fixed timing pattern for the plannedcommunications, by which the equidistance of the samples can be ensured.

The disadvantageous factor in this known communication method is,however, that this equidistance is no longer guaranteed in the case of adisturbed communication session. According to EN 50 170, in the case ofa disturbed communication session, this session is repeated between once(1 time) and fifteen times (15 times) in the same bus cycle. This leadsto the duration of the bus cycle being extended by the durationresulting from the repetition of the disturbed communication session.Equidistance of individual communication sessions over a number of buscycles—as is required, in particular, for critical sampled-data controlsystems—cannot be guaranteed with the known communication method in thecase of communication and/or transmission disturbances.

The reason for this is that each communication session which is executedafter the disturbed communication session in time in the bus cycle isoffset in time in comparison with a “normal” bus cycle, i.e. a bus cyclewithout disturbed communication session. This also lowers the quality ofa sampled-data control system. In the extreme case, even the stabilityof the sampled-data control system can be put in question.

SUMMARY OF THE INVENTION

One aspect of the invention is based on the object, therefore, ofspecifying a communication method by which equidistant sampling of ameasurement value of the technical process is also still possible in thecase of transmission disturbances.

For this purpose, it is provided in a method for cyclic communicationbetween communication stations provided for controlling or monitoring atechnical process, via a bus, in which communication sessions plannedfor the communication stations are executed during in each case one buscycle of predeterminable duration, that, in the case of a disturbedcommunication session, its retransmission is planned for a subsequentbus cycle and that the disturbed communication session is acknowledgedwith a special acknowledgement code. According to an alternative, noretransmission of the disturbed communication session takes place aftera disturbed communication session which is acknowledged with a specialacknowledgement code. This can be tolerated since, in the case of acyclic communication, a retransmission is replaced by the communicationsession of the next cycle. In this arrangement, it is possible to adjustwhether no retransmission at all, one, two etc. retransmissions of thedisturbed communication session are to take place.

The advantages achieved by the method relate, in particular, to theduration of the bus cycle with the disturbed communication sessionremains unaffected by retransmission and/or correction measures. This isachieved by displacing a retransmission of a disturbed communicationsession into a subsequent bus cycle. In the case of an immediateretransmission of a disturbed communication session in the same buscycle, the starting time of all communication sessions following thedisturbed communication session in the bus cycle is displaced withreference to the bus cycle. Equidistance of these displacedcommunication sessions over a number of bus cycles would no longer beguaranteed.

If values which are included in a sampled-data control system aretransmitted by the displaced communication sessions, the samplingfrequency of the sampled-data control systems concerned is no longerconstant which is accompanied by a deterioration in the quality of thesampled-data control system and possibly even instability of thesampled-data control system. By displacing the retransmission of adisturbed communication session into a subsequent bus cycle, thestarting times of the subsequent communications essentially remainunaffected. Thus, the equidistance of the individual communicationsessions is also guaranteed over a number of bus cycles so that thecommunication method is also suitable for critical sampled-data controlsystems.

So that no retransmission and/or correction measures take place in thebus cycle with the disturbed communication sessions, the disturbedcommunication session is acknowledged with a special acknowledgementcode. This leads to the omission of, in particular, transmission and/orcorrection measures so that the remaining communication sessions plannedfor the bus cycle can be executed.

If further disturbed communication sessions occur in the same bus cycle,the remaining disturbed communication sessions are also dealt with likethe first disturbed communication session. i.e. its retransmission, too,is displaced into a subsequent bus cycle.

So that the retransmission of a disturbed communication session in asubsequent bus cycle does not impair the equidistance of thecommunication sessions planned for this subsequent bus cycle, theretransmission of the disturbed communication session takes placefollowing the communication sessions planned for this bus cycle. Due tothe fact that the retransmission of the disturbed communication sessiontakes place following the communication sessions planned for this buscycle, their starting times, based on the bus cycle, remain unaffected.The equidistance of the communication sessions over a number of buscycles is thus guaranteed even during the retransmission of thedisturbed communication session.

The retransmission of the disturbed communication session isadvantageously planned for a bus cycle immediately following the buscycle with the disturbed communication session. As a result, the timedifference between the disturbed communication session, the time of theplanned execution and the retransmission, the time of the actualexecution, remains as small as possible.

If the disturbed communication session is acknowledged with the specialacknowledgement code like a faultless communication session, a delay inthe execution of the communication sessions following in the bus cycleis avoided. It is only in the case of a communication session which hasbeen executed faultlessly or acknowledged as faultless that theexecution of the communication sessions following in the bus cycle canbe continued. Due to the acknowledgement of the disturbed communicationsession with the special acknowledgement code, this session cannot bedistinguished from a faultless communication session. First, thebeginning of the execution of the communication sessions following inthe bus cycle is not delayed by any evaluation of a specialacknowledgement code.

The special acknowledgement code is advantageously converted into anormal acknowledgement code. This reduces the number of possibleacknowledgement codes to be checked so that, in spite of the existenceof an acknowledgement code which is additional in fact, the complexityof the analysis of the possible acknowledgement codes is not increased.The normal acknowledgement code is the acknowledgement code whichidentifies a communication session which has been faultlessly executed.The special acknowledgement code can be converted into a normalacknowledgement code, e.g. by so-called “masking”. If, e.g., the specialacknowledgement code only differs from the normal acknowledgement codedue to the first bit being set, the masking can begin by a logical ANDoperation on the special acknowledgement code, e.g. with the hexadecimalvalue “7FFF”.

If a retransmission counter is provided by which the number ofretransmissions of the disturbed communication session is counted, itcan be determined at any time how long the disturbance of thecommunication session already exists.

If a limit value is provided for the retransmission counter and anyretransmission of the disturbed communication session is acknowledgedwith a fault acknowledgement code after the limit value has beenreached, a disturbance of a communication session which lasts “too long”can be detected.

The limit value from which each retransmission of the disturbedcommunication session is acknowledged with a fault acknowledgement codecan be suitably predetermined. Thus, the time interval after the passageof which a disturbance of a communication session exists for “too long”can be predetermined. The limit value can be preferably predeterminedindividually for each communication session. Thus, the limit value for acommunication session which, e.g., supplies data for a critical firstsampled-data control system can be set to a different limit value thanthat of a communication session by which only record data aretransmitted.

If the retransmission counter can be read out by the communicationstation involved in the disturbed communication session, it candetermine at any time whether and possibly for how long a communicationsession is disturbed.

If at least one threshold value is provided for the retransmissioncounter and planned measures are initiated by the communication stationreading out the retransmission counter when the threshold value isreached, it is also possible to respond to the disturbance of thecommunication session in a suitable manner even before the limit valueis reached. For example, when a first threshold value is reached, acorresponding note can be output, for instance on a screen or printer,when a second threshold value is reached a visual or oral warning can beinitiated and when a third threshold value is reached, fault-limitingmeasures can be initiated. The fault-limiting measures can relate to,e.g., attempting to establish the communication session on a differentpath—for example with a redundant bus—, or placing the technicalprocess—possibly the part process controlled by the communicationstation—into a safe state.

Advantageously, a structure is provided in a memory of at least onecommunication station which has for each communication session one fieldin which the value of the retransmission counter is stored in a firstposition, the limit value is stored in a second position and the atleast one threshold value, together with a reference to the measure tobe initiated when the threshold value is reached, is stored in a thirdposition. The structure is used for the compact storage of the essentialdata which are provided for executing the communication method. Storinga reference to the measure to be initiated when the threshold value isreached makes it possible to directly call up a program routine in whichthe measure is programmed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 shows communication stations communicatively connected via a busfor controlling a technical process,

FIGS. 2 a and 2 b show communication sessions between individualcommunication stations,

FIG. 3 shows faultlessly executed communication sessions during two buscycles,

FIG. 4 shows a message retransmission as a consequence of a disturbedcommunication session,

FIG. 5 shows a message retransmission in a later bus cycle,

FIG. 6 shows a layout of a data structure, and

FIG. 7 shows a flowchart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

FIG. 1 shows communication stations 1, 2, 3, the communication stationdesignated by the reference symbol 1 being a so-called master 1—e.g. astored-program control—and the communication stations designated by thereference symbols 2, 3 being so-called slaves 2, 3—e.g. decentralizedperipheral devices. The communication stations 1, 2, 3 arecommunicatively connected to one another via a bus 4.

The master 1 is a communication station which has an active transmitauthorization on the bus 4. A slave 2, 3, in contrast, only transmitsafter having first been addressed by the master 1. Slaves 2, 3 are,therefore, lacking the active transmit authorization because they onlyrespond to a request (being addressed) by the master 1.

The communication stations 1, 2, 3 are provided for controlling ormonitoring a technical process 50 shown diagrammatically. The technicalprocess 50 comprises a reactor 51 with an inlet 52 and an outlet 53. Thereactor 51 is fed by the inlet 52. A reagent 54 leaves the reactor 51via the outlet 53. The inlet 52 is controlled by a valve 55. A fillinglevel meter 56 is used for determining a filling level 54′ of thereactor 51.

A simple control and/or monitoring (automation) of the technical process50, which will be used by way of an example in the text which follows,relate to an assumption that the valve 55 is controlled with a view to aconstant filling level 54′ of the reactor 51.

FIGS. 2 a and 2 b show the data exchange or, respectively, communicationsessions 12, 21, 13, 31 between communication stations 1, 2, 3 for thisautomation. The data exchange takes place under control of a program 6,stored in a memory 5, which is executed by the master 1. For thispurpose, the program 6 comprises a task 7, 7′ which is executed in afixed timing pattern and is called up, e.g., every 500 ms. With eachexecution of the task 7, 7′, a data exchange 12, 21 and 13, 31,respectively, takes place between the master 1 and the slave 2 (FIG. 2a) and between master 1 and slave 3 (FIG. 2 b).

In accordance with a planned communication session, a data exchangetakes place between the relevant communication stations 1, 2, 3. Thedata exchange takes place by a message 12, 21, 13, 31. In the text whichfollows, therefore, the terms communication/communication session andmessage are used synonymously. If the master 1 addresses a slave 2, 3,this takes place by a message 12, 13. The slave 2, 3, in turn, respondsto this stimulus with a message 21, 31.

FIG. 2 a shows a message 12 sent by the master 1 to the slave 2. Themessage 12 comprises e.g. output and control data in the form of digitaland/or analog values, e.g. a maximum value for the filling level 54′.However, the message 12 also causes the slave 2 to send its input datato the master 1 in a message 21. The message 21 thus contains, inparticular, a value representing the filling level 54′ of the reactor 51which is picked up by the filling level meter 56. The message 12implicitly issues to the slave 2, which is not actively authorized totransmit, an authorization for transferring the data requested by themaster 1. This is done by the message 21.

FIG. 2 b shows a message 13 by which the master 1 sends to the slave 3the respective output data. The message 13 comprises output and/orcontrol data in the form of digital or analog values, among them also avalue predetermining the position of the valve 55. The message 13 causesthe slave 3 to send its input data, e.g. also a value representing theactual volume flowing through the inlet 12, to the master 1 in a message31.

A message 12, 13, 21, 31 is always acknowledged 12′, 13′, 21′, 31′. Thefaultless undisturbed transmission of a message 12, 13, 21, 31 isacknowledged by a normal acknowledgement which specifies that thecommunication 12, 13, 21, 31 has been executed faultlessly. The normalacknowledgement comprises, e.g., a normal acknowledgement code (notshown) with the value “0x00”.

Automation of the technical process 50 (FIG. 1) requires a closed-loopcontrol for keeping the filling level 54′ of the reactor 51 constant.Since the message 21 comprises the measurement value of the fillinglevel 54′ and the latter is thus “sampled” only when the slave 2transmits the corresponding message 21 to the master 1, this is asampled-data control system. The quality/stability of such asampled-data control system is mainly determined by the time intervalbetween two samples of a process parameter (in this case filling level54′). To achieve stable closed-loop control, the basic mathematicalcontrol models require sampling at equidistant times.

FIG. 3 diagrammatically shows the times t1 _(n), t2 _(f), t1 _(n+1), t2_(n+1) cluttered along a time axis, at which the messages 21, 13 aretransmitted to and from the corresponding slave 2, 3.

The slave 2 transmits the current filling level 54′ of the reactor 51 tothe master 1 with the message 21. At times t1 _(n), t1 _(n+1), theprocess parameter “filling level” 54′ is thus sampled. The new positionof valve 55 is predetermined by master 1 for slave 3 with message 13.Messages 12 (FIG. 2 a) and 31 (FIG. 2 b) are not shown for reasons ofclarity.

To obtain stable control of the filling level 54′, it is important thatthe time interval between two successive times t1 _(n), t1 _(n+1), atwhich the filling level 54′ is sampled, remains constant. The timedifference between time t1 _(n) and t2 _(n+1), i.e. the picking up ofthe filling level 14′ and the outputting of the resultant control valueto the process 50, represents a dead time which can be easily taken intoconsideration and compensated for mathematically.

In the case of an undisturbed faultless communication, the equidistancebetween two successive times t1 _(n), t1 _(n+1) is guaranteed by thefixed timing pattern in which the task 7, 7′ (FIG. 2 a, 2 b) is executedunder the control of which the communication sessions are executed.

The time interval Δt designates the duration of a bus cycle, the termsbus cycle and duration of a bus cycle being used synonymously in thetext which follows. The duration of a bus cycle (bus cycle time) Δt isconstant. During a bus cycle Δt, all planned communication sessions areexecuted. If the starting time of task 7, 7′ (FIG. 2 a, 2 b), under thecontrol of which the data exchange 21, 13 between the master 1 and theslaves 2, 3 is executed, falls into a bus cycle Δt, the communicationsessions 21, 13 belong to the communication sessions planned for thisbus cycle Δt.

FIG. 3 shows two bus cycles Δt which in each case comprise thecommunication sessions 21, 13. These two bus cycles Δt do not follow oneanother directly in time—indicated by the broken timeline. Between thetwo bus cycles shown, one or more other bus cycles are executed which donot include the communication sessions 21, 13.

FIG. 4 shows the effect of a disturbed communication session 21 ^(s) onthe duration of a bus cycle Δt. In the second bus cycle Δt_(s) shown, adisturbance which impairs the execution of the communication session 21occurs at time t1 _(n+1). The disturbed communication session 21 ^(s) isacknowledged with a fault acknowledgement 21″. Each acknowledgement 21′,21″ comprises an acknowledgement code 21′, 21″ so that this faultacknowledgement 21″, too, comprises a fault acknowledgement code 21″which unambiguously species the type of the fault. The disturbedcommunication relation 21 ^(s) is followed by a retransmission 21 ^(w)of the disturbed communication session. The first retransmission 21 ^(w)of the disturbed communication session cannot be executed faultlessly,either, and is, therefore, acknowledged with a fault acknowledgement21″. It is only after the second retransmission 21 ^(w) that it can befaultlessly executed. The faultlessly executed second retransmission 21^(w) is correspondingly acknowledged with a normal acknowledgement 21′.The normal acknowledgement 21′ comprises a normal acknowledgement code21′ which specifies the faultless execution.

Overall, the communication session 13 is thus executed correspondinglylater in time, namely only at time t2′_(n+1). In the case of a bus cycleΔt (FIG. 3) which is not encumbered by a disturbed communicationsession, in contrast, the execution occurs at time t2 _(n+1). The timeoffset by which the communication session 13 is executed latercorresponds to the duration of the two retransmissions 21 ^(w) of thedisturbed communication session 21 ^(s).

The new value for the position of the valve 55 (FIG. 1) is transmittedwith the message 13. The intervention in the closed-loop control is thusdelayed, i.e. no longer at equidistant times so that it may no longer bepossible to keep the filling level 54′ (FIG. 1) constant. The moredynamic the controlled system the stronger this will affect the qualityof the closed-loop control. In the extreme case, even the stability ofthe closed-loop control can be put in question. Furthermore, theduration Δts of the bus cycle with the disturbed communication 21 s isextended in comparison with the duration Δt of the bus cycle with thefaultlessly executed communication 21. This leads to communicationsessions executed in each bus cycle also no longer being equidistant.

FIG. 5 shows how equidistance is guaranteed even in the case of adisturbed communication session 21 ^(s). It shows two bus cycles Δt_(s),Δt immediately following one another. In the first bus cycle Δt_(s), adisturbance 21 ^(s) occurs. A third bus cycle Δt is executed later intime—indicated by the broken timeline.

Analogously, a disturbance 21 ^(s) occurs during the execution of thecommunication session 21—at time t1 _(n+1). The disturbed communicationsession 21 ^(s) is acknowledged with a special acknowledgement 21′ whichcomprises a special acknowledgement code 21′ like a faultlessly executedcommunication session. i.e., the special acknowledgement code 21′ isconverted into a normal acknowledgement code 21′ or the specialacknowledgement code 21′ is evaluated like a normal acknowledgement code21′. Thus, there is no immediate retransmission of the disturbedcommunication session 21 ^(s) in the same bus cycle Δt_(s). On thecontrary, the retransmission of the disturbed communication session 21^(s) is planned for the next bus cycle Δt.

Thus, there is no time offset in the transmission of the new value forthe position of the valve 55 by the message 13 even in the bus cycleΔt_(s) with the disturbed communication session 21 ^(s). The message 13is still executed at time t2 _(n+1) as before.

The retransmission 21 ^(w) of the disturbed communication session 21^(s) is executed at time t1 _(n+1) ^(w) in the bus cycle Δt followingimmediately. For such a retransmission 21 ^(w), a special communicationsection t30 is provided at the end of each bus cycle Δt. Plannedcommunication sessions 13, 21 are executed at the beginning of each buscycle Δt in a normal communication section t20.

The number of retransmissions 21 ^(w) of a disturbed communicationsession 21 ^(s) is counted in a retransmission counter 111 according toFIG. 6. If the retransmission counter 111 reaches a predetermined limitvalue 112, each retransmission 21 ^(w) of the disturbed communicationsession 21 ^(s) is acknowledged with a fault acknowledgement (not shown)which comprises a fault acknowledgement code, when the limit value 112is reached. Thus, permanently disturbed communication sessions can berecognized as such and the communication station which can no longer bereached can be flagged as failed.

Dividing a bus cycle Δt into the normal communication section t20 andthe special communication section t30 leads to a decoupling betweenmessages 13, 21 of correspondingly planned communication sessions andmessage retransmissions 21 ^(w) due to disturbed communication sessions21 ^(s). The message retransmission 21 ^(w) in bus cycle Δt—i.e. in thesecond bus cycle in FIG. 5—is decoupled from planned communicationsessions (shown dashed) to be executed in the normal communicationsection t20.

Since the duration of the bus cycle Δt is predetermined and constant,either the duration of the normal communication section t20 or theduration of the special communication section t30 is also predetermined.If the duration of the normal communication section t20 ispredetermined, the fixed bus cycle time Δt will produce the duration ofthe special communication section t30 and vice versa. The duration ofthe normal communication section t20 and special communication sectiont30 is dimensioned in such a manner that at least one messageretransmission 21 ^(w) can take place during the special communicationsection t30.

FIG. 6 shows a structure 100 which is provided in the memory 5 (FIG. 2a, 2 b) of a communication station 1, 2, 3. The structure 100 has aseparate field 110, 120, 130 for each communication session—at least foreach communication session in which the relevant communication station1, 2, 3 is involved. In each field 110, 120, 130, the value of theretransmission counter 111, 121 is stored in a first position, the limitvalue 112, 122 is stored in a section position and the at least onethreshold value 114, 124, together with a reference 115, 125 to themeasure to be initiated when the threshold value 114, 124 is reached, isstored in a third position 113, 123.

The structure is used for compact storage of the essential data whichare provided for executing the communication method. Storing a reference115, 125 to the measure to be initiated when the threshold value 114,124 is reached enables a program routine to be called up directly inwhich the measure is programmed.

The omission points “ . . . ” in the structure 100, on the one hand,and, on the other hand, in the field 130 indicate that the structure cancomprise other fields 110, 120, 130, depending on the number ofcommunication sessions, and that the field 130, like any other fields,basically has the same layout as the field 110, 120.

FIG. 7 shows in a flowchart an algorithm for essential aspects of thecommunication method, which begins in step 1001 if the retransmission 21^(w) of the disturbed communication session 21 ^(s) could not beexecuted faultlessly either. In step 1010, the retransmission counter11, 121 (FIG. 6) is incremented with each retransmission 21 ^(w) of adisturbed communication session 21 ^(s) (FIG. 5).

In step 1020, a check is made whether the retransmission counter 11, 121has reached the limit value 112, 122 (FIG. 6). If this is so, the systembranches to step 1040 and the retransmission 21 ^(w) of the disturbedcommunication session is acknowledged with a fault acknowledgement code21″ (FIG. 4). After execution of step 1040, the algorithm is ended instep 1002. If it is found in step 1020 that the retransmission counter11, 121 has not yet reached the limit value 112, 122, the algorithm iscontinued in step 1030.

In step 1030, the retransmission 21 ^(w) of the disturbed communicationrelation 21 ^(s) is acknowledged with the special acknowledgement code21′ (FIG. 5). The acknowledgement of the retransmission 21 ^(w) of thedisturbed communication session 21 ^(s) with the special acknowledgementcode 21′ has the effect that a failed retransmission 21 ^(w) of thedisturbed communication session 21 ^(s) is also treated like afaultlessly executed communication and a next retransmission 21 ^(w) isplanned for a following bus cycle.

In step 1050, a check is made whether the retransmission counter 111,121 has reached the threshold value 114, 124 (FIG. 6)—possibly one of anumber of threshold values. If this is so, the system branches to step1060 and a measure is triggered, the reference (address) of which isstored at position 115, 125 (FIG. 6). If, e.g., the limit value 112, 122is set to the value “20”, the threshold value 114, 124 can be set e.g.to value “10”. I.e. after ten unsuccessful retransmissions 21 ^(w) of adisturbed communication session 215, the threshold value 114, 124 isreached and a corresponding measure 115, 125 can be initiated. Thismeasure 115, 125 can include, e.g., outputting a warning message on adisplay device (not shown) in order to indicate the disturbedcommunication session 21 ^(s). The actual measure is implemented asprogram routine (subroutine).

As a reference 115, 125, its start address is stored at position 115,125. When the threshold value is reached, the measure 115, 125 can betriggered directly on the basis of the stored reference 115, 125. Afterstep 1060 has been executed, the algorithm is ended in step 1002. If itis found in step 1050 that the retransmission counter 111, 121 has notyet reached the threshold value 114, 124, the algorithm is endedimmediately in step 1002.

The algorithm is started every time in step 1001 even if theretransmission 21 ^(w) of the disturbed communication session 21 ^(s)could not be executed faultlessly. If, in contrast, the firstretransmission 21 ^(w) of the disturbed communication session 21 ^(s)has already been faultlessly executed, this is acknowledged with thenormal acknowledgement code 21′ analogously to step 1030. In this case,evaluation of the limit or threshold value 111, 121 and 114, 124 is notrequired.

Thus, a method for cyclic communication between communication stations1, 2, 3, provided for controlling or monitoring a technical process 50,via a bus 4, is specified in which communication sessions 12, 13, 21,31, which have been planned for the communication stations 1, 2, 3, areexecuted during in each case one bus cycle of predeterminable durationΔt. In the case of a disturbed communication session, its retransmission21 ^(w) is planned for a subsequent bus cycle and the disturbedcommunication session is acknowledged with a special acknowledgementcode.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1. A method for cyclic communication via a bus, between communication stations provided for controlling a technical process, comprising: allotting repeated bus cycles to communication sessions such that during each bus cycle, each of the communication stations has a planned communication session, each of the bus cycles having a predetermined duration; in the case of a disturbed communication session, planning retransmission of the disturbed communication session for a bus cycle subsequent to the bus cycle in which the disturbance occurred; and acknowledging the disturbed communication session with a special acknowledgement code.
 2. The method as claimed in claim 1, wherein for the bus cycle subsequent to the bus cycle in which the disturbance occurred; the communication sessions allotted to the bus cycle occur before retransmission of the disturbed communication session.
 3. The method as claimed in claim 2, wherein retransmission of the disturbed communication session is planned for a bus cycle immediately subsequent to the bus cycle in which the disturbance occurred.
 4. The method as claimed in claim 3, wherein successful communication sessions are acknowledged with normal acknowledgment codes, and the special acknowledgement code is produced in the same manner as the normal acknowledgement codes.
 5. The method as claimed in claim 4, wherein the special acknowledgement code is converted into a normal acknowledgement code.
 6. The method as claimed in claim 5, wherein if a first retransmission of the disturbed communication session is not successful, further retransmissions of the disturbed communication session are planned, and the number of retransmissions of the disturbed communication session is counted by a retransmission counter.
 7. The method as claimed in claim 6, wherein the retransmission counter has a limit value, and after the limit value has been reached, a fault acknowledgment code is used to acknowledge any subsequent unsuccessful retransmissions of the disturbed communication session.
 8. The method as claimed in claim 7, wherein the retransmission counter is read at least by the communication station involved in the disturbed communication session.
 9. The method as claimed in claim 8, wherein the retransmission counter has a threshold value which is below the limit value, and when the number of unsuccessful retransmissions reaches the threshold value, the communication system undertakes alternate measures for the disturbed communication session.
 10. The method as claimed in claim 9, wherein a memory is provided in at least one of the communication stations, the memory has first field to store a current value for the retransmission counter, the memory has a second field to store the limit value, the threshold value, and information regarding the alternate measures.
 11. The method as claimed in claim 1, wherein retransmission of the disturbed communication session is planned for a bus cycle immediately subsequent to the bus cycle in which the disturbance occurred.
 12. The method as claimed in claim 1, wherein successful communication sessions are acknowledged with normal acknowledgment codes, and the special acknowledgement code is produced in the same manner as the normal acknowledgement codes.
 13. The method as claimed in claim 1, wherein the special acknowledgement code is converted into a normal acknowledgement code.
 14. The method as claimed in claim 1, wherein if a first retransmission of the disturbed communication session is not successful, further retransmissions of the disturbed communication session are planned, and the number of retransmissions of the disturbed communication session is counted by a retransmission counter.
 15. The method as claimed in claim 14, wherein the retransmission counter has a limit value, and after the limit value has been reached, a fault acknowledgment code is used to acknowledge any subsequent unsuccessful retransmissions of the disturbed communication session.
 16. The method as claimed in claim 15, wherein the limit value is set individually for each communication session.
 17. The method as claimed in claim 15, wherein the retransmission counter has a threshold value which is below the limit value, when the number of unsuccessful retransmissions reaches the threshold value, the communication system undertakes alternate measures for the disturbed communication session, a memory is provided in at least one of the communication stations, the memory has first field to store a current value for the retransmission counter, the memory has a second field to store the limit value, the threshold value, and information regarding the alternate measures.
 18. The method as claimed in claim 14, wherein the retransmission counter is read at least by the communication station involved in the disturbed communication session.
 19. The method as claimed in claim 14, wherein the retransmission counter has a threshold value, and when the number of unsuccessful retransmissions reaches the threshold value, the communication system undertakes alternate measures for the disturbed communication session.
 20. The method as claimed in claim 1, wherein the special acknowledgement code avoids an immediate repeat of the disturbed communication session. 